We will utilize molecular and genetic techniques to examine the structure, function, and developmental regulation of the Drosophila gene encoding myosin heavy chain (MHC). This gene is unusual in that it is not a member of a multigene family. Myosin genes from other organisms constitute multigene families which code for isoforms of the MHC protein. The Drosophila gene however, encodes transcripts of three sizes, one of which is specific to embryos and one to pupae. This suggests that transcripts encoding isoforms of MHC may be derived from this single gene. We will examine the differences between the three MHC transcripts and determine whether they encode different isoforms. This analysis will indicate whether MHC isoforms are necessary and if so, what regions of the MHC protein perform tissue- or stage-specific functions. We will also determine whether these three transcripts accumulate in a tissue-specific manner and if unrelated MHC genes are expressed in visceral muscle and non-muscle tissues. In order to study the tissue- and stage-specific functions of the skeletal muscle MHC gene, we will utilize both in vivo and in vitro mutagenesis. MHC gene lethal mutations will be produced and mutants will be compared as to gene lesion and tissue-specific effects. Mutant MHC genes will be constructed in vitro and used to transform Drosophila embryos by injection. With this technique, we will be able to examine the function of transcript-specific exons and the DNA sequences necessary for stage-specific transcript production.